The present invention relates to a process for producing cellulose acetate and more specifically relates to a process for producing cellulose acetate comprising the step of the reaction using an acetate as an acetylating agent.
Cellulose acetate has been used for a variety of purposes such as fibers for clothing, tobacco filter chips, plastics, films and coatings, and the output of cellulose acetate is the most of cellulose derivatives, hence cellulose acetate is industrially important compound.
A cellulose acetate film has been used as the support of a photographic material on one hand, due to its optical isotropy it has also been used in recent years for a liquid crystal display device which is extending the market. A cellulose acetate film is in general produced by a solvent casting method. The degree of acetylation of cellulose acetate is closely related with the mechanical strength and the durability of the film obtained, and it is known that as the degree of acetylation of cellulose acetate lowers, the qualities which are required of the optical film such as the modulus of elasticity and the dimensional stability of the film are reduced. For ensuring satisfactory qualities of the product, cellulose acetate should have the degree of acetylation of 58% or more, preferably 59% or more. Cellulose acetate having the degree of acetylation of 58% or more is generally classified as cellulose triacetate (triacetyl cellulose, i.e., TAC). In a solvent casting method, a solution containing cellulose acetate dissolved in a solvent (a dope) is cast-coated on a support and the solvent is evaporated to thereby form a film.
It is important that a solvent for use in a solvent casting method not only has high solubility but also easily evaporates and scarcely remains in a film. Methylene chloride has so far been used as a solvent for this purpose but the use of methylene chloride has come to be restricted from the environmental point of view. Therefore, searching out an alternative solvent is now an important problem, and a method of using an organic solvent containing acetone as a main component and dissolving cellulose acetate by cooling to thereby obtain a cellulose acetate film (JP-A-9-95544 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)), and a method of producing a cellulose acetate film with an organic solvent containing acetone as a main component under high pressure condition of from 1 to 500 MPa (JP-A-11-21379) have been suggested.
Representative industrial production method of cellulose acetate includes a method using an acetic anhydride as an acetylating agent, an acetic acid as a diluent and a sulfuric acid as a catalyst (e.g., JP-A-5-239101 can be referred to). This method comprises the steps of (1) a pretreatment process of disaggregating and cracking a pulp material and then spraying acetic acid and mixing with the pulp material, (2) an acetylation process of reacting the pretreated pulp by using an acetylating agent, a diluent and a catalyst and, if necessary, (3) an aging process of hydrolyzing the cellulose acetate to make cellulose acetate having a desired degree of acetylation, and (4) a post treatment process of precipitating and dissociating the cellulose acetate having undergone the aging reaction from the reaction solution, refining, stabilizing and drying.
It has been tried to produce solutions containing cellulose acetate produced by the above-described ordinary method with solvents other than methylene chloride. However, according to the above method of using an organic solvent containing acetone as a main component (disclosed in JP-A-9-95544), a cellulose acetate solution (a dope) excellent in storage stability cannot be obtained from cellulose acetate having high degree of acetylation, preferably 61% or so, and such is the state of things that cellulose acetate which can be used has low degree of acetylation. Further according to the method disclosed in JP-A-11-21379, such severe pressure as 100 MPa at 25xc2x0 C. is required.
An object of the present invention is to provide a process for producing cellulose acetate which is suitable for preparing a cellulose acetate solution (a dope) by using an organic solvent other than methylene chloride under more moderate conditions.
The above object of the present invention has been accomplished by the following means.
(1) A process for producing cellulose acetate which comprises reacting cellulose as a raw material with an acetylating agent, wherein the acetylating agent is an acetate.
(2) The process for producing cellulose acetate as described in the above item (1), wherein a solid acid catalyst is used in the reaction as a catalyst.
(3) The process for producing cellulose acetate as described in the above item (1) or (2), wherein the acetate is methyl acetate or ethyl acetate.
(4) The process for producing cellulose acetate as described in the above item (1), (2) or (3), wherein the reaction for acetylation is performed under the condition of pressure of 0.2 MPa or more.
The cellulose used as a raw material in the present invention is more specifically cotton linter or wood pulp.
Cotton linter and wood pulp may be used by mixture. Wood pulp is more inexpensive and economical than cotton linter.
An ordinary process for producing cellulose acetate by using wood pulp is described below. In the first place, wood pulp is treated with a cracking machine to make fluffy cracked pulp. In the pretreatment of the cracked pulp, it is preferred for 100 g of the cracked pulp to be sprayed and mixed with from about 400 to about 1,000 g of a glacial acetic acid, in some cases containing a catalyst (e.g., a sulfuric acid), and stirred in a closed vessel with a stirrer at temperature of from 20 to 80xc2x0 C. for from 0.5 to 4 hours or so. Such pretreatment is performed for the purpose of swelling the cellulose fiber to ensure smooth and uniform acetylation reaction to the cellulose.
Subsequently, from 150 to 800 g, preferably from 200 to 400 g, of an acetic anhydride as an acetylating agent is added to 100 g of the cracked pulp, stirred and mixed. The content in the reaction vessel generates heat due to the reaction. The temperature is adjusted so as to increase at an almost constant rate over 20 to 60 minutes, and the final temperature of from 50 to 80xc2x0 C. is retained for a period of from 3 to 20 minutes to thereby acetylate the cellulose.
In the present invention, acetate (preferably those having from 3 to 18 carbon atoms, more preferably from 3 to 10 carbon atoms, e.g., methyl acetate and ethyl acetate) is used as an acetylating agent. Acetate is used preferably in an amount of 100 wt % or more based on the cracked pulp, more preferably from 100 to 20,000 wt %, and still more preferably from 300 to 1,000 wt %, with serving also as a diluent at the same time.
It is particularly preferred in the present invention to make an acetate solution without using other organic solvents and diluents but, if necessary, other organic solvents can be used, e.g., a halogen-based solvent, a hydrocarbon-based solvent, an alcohol-based solvent and an acetic acid can be used. When these solvents are used, the use amount is preferably from 1 to 1,000 wt % based on the cracked pulp.
As a catalyst, a sulfuric acid, a hydrochloric acid, a hydrobromic acid, and a perchloric acid are preferably used in the present invention, more preferably a sulfuric acid. A solid acid catalyst can also be preferably used in the present invention. Specific examples of solid acid catalysts include, e.g., an acid ion exchange resin, an acid clay, and a silica alumina. The solid acid catalyst has been improved variously in recent years (e.g., the solid acid catalysts disclosed in JP-A-5-261298, JP-A-6-182212, JP-A-6-327976, JP-A-7-47280, JP-A-7-171397, JP-A-8-57321, JP-A-8-126842, JP-A-8-126843, JP-A-9-75735, JP-A-9-103681, JP-A-10-809727, JP-A-11-57478, JP-A-11-188261, JP-A-11-244701, JP-A-11-309375, etc.). Although the solid acid catalysts disclosed in the above patents can be used in the present invention, it is preferred to use an acid ion exchange resin. As an acid ion exchange resin, Amberlyst 15 (a trade name), Amberlite IRP64 (a trade name), AMBERJET 1200 (a trade name), and Nafion (a trade name) are preferably used. A catalyst is used in an amount of preferably from 0.1 to 2,000 wt %, more preferably from 1 to 1,000 wt %, and still more preferably from 10 to 100 wt %, based on the cracked pulp. When a solid acid catalyst is used, it can be dissociated by filtration and the like after the reaction.
The acetylation reaction is preferably performed in an autoclave under high pressure of 0.2 MPa or more, more preferably from 4 to 30 MPa.
The critical point of an acetic acid is 5.79 PMa at 319xc2x0 C., the critical point of an isobutyl acetate is 3.16 MPa at 288xc2x0 C., the critical point of an ethyl acetate is 3.83 MPa at 250xc2x0 C., the critical point of a butyl acetate is 3.11 MPa at 306xc2x0 C., the critical point of a propyl acetate is 3.33 MPa at 276xc2x0 C. and the critical point of methyl acetate is 4.69 MPa at 233xc2x0 C., and it is preferred to perform pretreatment and esterification in a subcritical or supercritical condition in the vicinity of critical points. If necessary, a carbon dioxide (critical point is 7.38 MPa at 31xc2x0 C.) may coexist in an autoclave.
The thus-obtained cellulose acetate solution (a dope) can be cast-coated on a support and the solvent is evaporated to thereby form a film. If necessary, a low molecular weight cellulose acetate can be removed.
The method of esterifying cellulose by using esters under high pressure can be applied not only to the manufacture of cellulose acetate but also to the manufacture of fatty acid cellulose such as cellulose propionate and cellulose butyrate.
According to the method of the present invention, cellulose acetate which is suitable for preparing a cellulose acetate solution (a dope) can be produced with an organic solvent other than methylene chloride under more moderate conditions. Further, an acetate solution of cellulose acetate can be obtained by a short process according to the present invention.